The present invention relates to a semiconductor device suitable for performing high speed switching, for example, an improved structure for transistor-transistor logic (TTL) circuits.
TTL circuit are most widely used for bipolar integrated circuits such as large scale integrated circuits (LSI) or very large scale integrated circuits (VLSI), for example because of its low power dissipation and faster speed of operation than diode transistor logic (DTL) circuits. Efforts have been made recently to discharge carriers stored in a base portion of an output transistor when it is switched from the ON state to the OFF state.
A typical TTL circuit is shown in FIG, 1, wherein the output transistor is designated by Tr.sub.1 and the multi-emitter input transistor by Tr.sub.2. This multi-emitter transistor Tr.sub.2 performs the circuits's logic function, with inputs being supplied to emitters of transistor Tr.sub.2. Resistor R.sub.L shown in FIG. 1 is provided for discharging minority carriers stored in the base region of the transistor Tr.sub.1 when the output of the TTL circuit is switched from low level, that is, when the transistor Tr.sub.1 is switched to high level or from ON state to OFF state. This serves to improve the speed of operation.
On the other hand, it is observed that, contrary to the case where the output is switched from low level to high level, the TTL circuit shown in FIG. 1 delays the operation of turning OFF transistor Tr.sub.4 which is ON when the output of the TTL circuit is switched from high to low level. In a transient period when the output changes from high to low level, by virtue of resistor R.sub.L, a current flows from resistor R.sub.1 to resistor R.sub.L via transistor Tr.sub.3, making the change of the collector voltage of the output transistor slower for the transition from high level to low level. Thus, resistor R.sub.L functions rather adversely for high speed switching of the output transistor from high level to low level.
It has been suggested to replace resistor R.sub.L of FIG. 1 by a transistor or a diode and resistance connected in series, as in U.S. Pat. No. Re. 27804. However, this proposal is not effective in solving the problem described above, and the transfer characteristics at the output level have not been significantly improved. In addition, providing a transistor or a diode and resistance takes up chip area beside involving additional steps to make them.